It is narrated from Sayyidah Aishah (may Allah be pleased with her) that the Prophet (peace and blessings be upon him) never urinated while standing from the time the Qur'an was revealed to him (peace and blessings be upon him).
(b) In the narration of Husayn bin Hafs it is: since Surah al-Furqan was revealed. Some say the words are "the Qur'an or al-Furqan".
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 491